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<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>

    let x = 10;
    let y = 20;

    x++; // x += 1; // x = x + 1; 11
    y--; // y -= 1; // y = y - 1; 19
  </script>
</body>

</html>